In wavelength division multiplexed (WDM) signal spectra, high-bandwidth modulation is imposed upon a series of wavelength-separated optical carriers. To characterize a WDM signal spectrum, accurate measurement of the modulated optical carriers (the signal component of the spectrum) and the noise within spectral intervals between the modulated optical carriers (the noise component of the spectrum) are needed. A trade-off in measurement accuracy exists when measurement bandwidth is relied upon to measure both the signal component and the noise component of the WDM spectrum. With a measurement bandwidth that is wide enough to accommodate the high bandwidth of the modulated optical carriers, the measurement sensitivity is typically too low to enable accurate measurement of the noise component of the spectrum. Alternatively, with a measurement bandwidth that is narrow enough to achieve sufficient measurement sensitivity to accurately measure the noise component of the spectrum, the measurement bandwidth is too low to accurately characterize the signal component of the spectrum. There is a need for a characterization method that provides high enough measurement sensitivity to accurately characterize the noise component of a WDM spectrum, and that also provides high enough measurement bandwidth to accurately characterize the signal component of the WDM spectrum.
A spectral characterization method constructed according to the preferred embodiment of the present invention accurately characterizes the signal component and the noise component of an applied signal spectrum, such as the spectrum of a wavelength division multiplexed (WDM) signal, that has multiple, spectrally-separated signal peaks.
The method includes detecting signal peaks within the signal spectrum and measuring the spectral content of that spectrum using a first measurement bandwidth that is sufficiently wide to encompass each of the signal peaks. This wide measurement bandwidth provides for accurate measurement of the signal component of the signal spectrum. A center wavelength and peak amplitude value for each signal peak in the signal spectrum are is designated based on the measured spectral content of the signal spectrum made using the first measurement bandwidth.
Spectral content of the signal spectrum is also measured using a second measurement bandwidth that is sufficiently narrow to substantially exclude capture of the signal peaks during measurement within the intervals of spectral separation between the signal peaks. Higher measurement sensitivity results in the spectral characterization of the noise component as the second measurement bandwidth is set narrower relative to the first measurement bandwidth. The noise component of the signal spectrum is accurately characterized when spectral content of the signal spectrum is measured using the second measurement bandwidth. Accurate characterization of the signal spectrum results from combining the characterized signal component measured using the wide, first measurement bandwidth, with the characterized noise component measured using the narrow, second measurement bandwidth. Signal-to-noise ratio at the center wavelength of each signal peak is calculated from the peak amplitude value for the signal peak and a noise level at the center wavelength extracted from the spectral content measured using the second measurement bandwidth at spectral offsets from the designated center wavelengths of each signal peak. The spectral characterization method can be implemented using an optical monochromator, optical spectrum analyzer or other spectral measurement instrument having an adjustable measurement bandwidth.